1. Field of the Invention
The invention relates to a superconducting hollow cable having an outer tube which has a circular inner cross-section and, consequently, a cylindrical inner wall. In addition, the superconducting hollow cable has a central cooling channel of a circular cross-section that is smaller than the inner cross-section of the outer tube. Arranged between the inner wall of the outer tube and the cooling channel are profiled superconducting wires. Such profiled superconducting wires comprise at least one superconductor (superconducting filament) and have a cross-sectional profile of a key stone as known for Roman stone bridges or for groin vaults.
2. Description of Related Art
Profiled superconducting cables having a cross-sectional profile of a key stone are known from the publication U.S. Pat. No. 6,675,623 B2. Further prior art is US 2003/0024 730. The known key-stone-shaped superconducting cables consist of superconducting wires of round cross-section stranded with one another and, after stranding, they are given a trapezoidal shape with the aid of profiling rollers.
The structure of a superconducting hollow cable of the Nuclotron type having round (standard variant, FIG. 16) or profiled (improved Nuclotron cable, FIG. 17) superconducting wires is known from the publication “Design and test of new hollow high current NbTi cable for fast ramped synchrotron magnets” Proc. EUCAS 2003, Sorrento, September 2003, H. Khodzhibagiyan et al. In the case of the known hollow cable, first a cooling channel is formed by means of providing a compact metallic inner tube of CuNi. Superconducting wires profiled in the longitudinal direction and having a key-stone-shaped cross-section are placed or wound on the outer wall of the inner tube. In order to fix the superconducting wires on the compact inner tube, NiCr wires are wound around the outside of the superconducting wires radially or in a helical arrangement. A Kapton tape wound over the NiCr wires forms a protective outer jacket. A fibreglass tape wound around the outer jacket ensures that the superconducting hollow cable is electrically insulated.
A superconducting hollow cable that is so structured has the advantage, amongst others, of being able to compensate for high dynamic loads as occur as a result of the Lorentz forces in fast-pulsed magnets and high magnetic fields. The heat sources brought about by those pulsating fields by means of eddy currents, hysteresis cycles, mechanical stress fields and other external influences (e.g. ion beams) can, in continuous operation, be very effectively cooled by the inner cooling channel by virtue of the compact inner tube. The relatively large cross-section of the compact inner tube, which has a smooth surface, allows a high throughput of coolant with low frictional resistance and, accordingly a low pressure drop in the two-phase helium stream. In addition, all the superconducting wires are arranged absolutely symmetrically and are consequently under identical operating conditions, as a result of which degradation of the critical current of the cable with increasing electromagnetic loading is avoided to a very large extent.
This arrangement of the superconducting wires furthermore results in low cable inductivity and consequently (inter alia) also in reduction of the energy stored.
Another cable type is, in general, preferred when the coil system to be produced is not intended to be operated in an extremely fast-pulsed regime but rather has to store a large amount of inductive energy which has to be delivered in high stochastic heat pulses of short duration. In that case, a continuous high cooling performance is secondary whereas the fastest possible heat transfer to the heat capacity of the helium cooling the superconducting wires in the millisecond range is primarily sought. For that task, the known hollow cables of the CICC type as shown in FIG. 14 were developed.
However, because those cables also increasingly have to overcome higher mechanical and thermal loads in practical use, further development thereof resulted in a high-current cable of the CICC type having as a result of additional constructional elements, complicated cooling with two basically different cooling circuits with supercritical helium in the outer cooling circuit and with two-phase helium in the central cooling channel.
That complicated structure not only has technical and, therefore, cost-influencing disadvantages but also automatically gives rise to a reduced average current density of the hollow cable. A cable of such a kind in accordance with the present prior art is shown in FIG. 15 and is known as a POLO cable (see, for example, “CHATS”, FzK, Karlsruhe, September 2002, L. Bottura et al.). It represents the starting point for the invention explained hereinbelow.
The problem of the invention is to overcome the disadvantages in the prior art and to provide a superconducting hollow cable which makes possible intensive cooling of the superconducting components and; in addition, makes available a high current density for a hollow cable of the CICC type with a compact and mechanically stable structure for the hollow cable. Solutions are to be provided as to how the disadvantages of the CICC cable with respect to the cable of the Nuclotron type can be overcome to a very large extent and how the advantages of both hollow cable types can be, as far as possible, in a new superconducting hollow cable.
The problem is solved by the subject-matter of the independent claims. Advantageous developments arise out of the dependent claims.